Chevrolet Light Duty Truck 1973, Service Manual

Page: 559 / 979

E N G IN E ELECTRICAL 6Y-31
centrifugal advance mechanism, plus a possible addi­
tional advance resulting from operation of the vacuum
advance mechanism. For example, an initial timing
advance of 5 degrees, plus a centrifugal advance of 10
degrees, makes a total of 15 degrees advance at 40 miles
an hour. If the throttle is only partly opened, an
additional vacuum advance of up to 15 degrees more
may be obtained, making a total of 30 degrees. When
the throttle is wide open there is no appreciable vacuum
in the intake manifold, so this additional advance will
not be obtained. All advance then is based on engine
speed alone and is supplied by the centrifugal advance
mechanism.
The vacuum advance mechanism is an economy device
which will increase fuel economy when properly used.
The driver who drives with wide-open throttle whether in
low or high gear will not obtain this additional advance
with its resulting increased fuel economy.
Cam Angle
The cam angle, often referred to as contact angle or
dwell angle, is the number of degrees of cam rotation
during which the distributor contact points remain
closed. See Fig. 8i. It is during this period of cam
rotation that the current in the primary winding
increases. Although the cam angle may not change, the
length of time the contacts remain closed becomes less
and less as the engine speed increases. At higher engine
speeds, the ignition coil primary current does not reach
its maximum value in the short length of time the
contacts are closed. In order to store the maximum
amount of energy obtainable on the coil, and
consequently obtain sufficient energy to fire the plug, it is
necessary to design a breaker lever assembly that will
operate properly at high speeds. The distributor is
equipped with a special-high rate-of-break cam and a
special high speed breaker lever which is capable of
following the cam shape at high speeds without
bouncing. The high rate-of-break cam separates the
contact points faster for each degree of rotation and
permits closing earlier, thus increasing cam angle. With
the speical cam and breaker lever combination, it is
possible to obtain the maximum cam angle and
consequently optimum ignition performance at high
speeds.
The point opening is the maximum distance that occurs
between the separated contacts as the cam rotates. If the
cam angle is properly set, the point opening most likely
will also be according to specifications. In some cases, it
may be necessary to measure point opening in addition
to cam angle to insure that the contacts are properly set.
A feeler gauge on new contacts, or a dial indicator on
used contacts may be used to measure point opening.
Ignition Condenser (Capacitor)
The capacitor consists of a roll of two layers of thin
metal foil separated by a thin sheet or sheets of
insulating material. (Fig. 9i). This assembly is sealed in a
metal can with a flat spring washer providing a tight
seal.
The high voltage induced in the coil primary causes the
capacitor plates to charge when the contacts first
LIGHT DUTY TRUCK SERVICE MANUAL